Local ionic motion in Ba1-xUxF2+2x(0<x⩽0.24)

Abstract

The authors report on 19F pulse-NMR experiments on the solid solutions Ba1-xUxF2+2x (7.7*10-<or=x<or=0.24) from 100 to 1250K. In T1 and T1 rho , evidence is found for fast local motions of fluorine interstitials around the U4+ dopant. The data are analysed in terms of a model containing fluorine-fluorine dipolar interactions as well as relaxation towards the paramagnetic U4+ centres. The theory for nuclear relaxation towards paramagnetic centres is modified to incorporate the fast local jumps, which are assumed to be restricted within the spin-diffusion barrier about the U4+ centres. The theory is further made applicable to high concentrations of these centres. At low x, the activation energy for the local process from NMR appears to equal the activation energy found from the conductivity at high x, at which concentrations the conduction path runs along percolating (UBa2F'i)x clusters. The transport parameters for the bulk-diffusive motions are in reasonable agreement with those deduced from the conductivity.